Biomedical Engineering Reference
In-Depth Information
reinforcement for advanced composites. This new class of materials, CNT
reinforced composites, aims at exploiting the increased strength of CNTs in
the absence of macroscopic defects, due to their small size (although atomic
scale defects exist), which should lead to high-quality structural materials.
Moreover, the tremendously increased surface area of CNTs due to their
nanosize produces novel effects, which are still being elucidated. CNTs have
been extensively used in polymeric, metallic and ceramic matrices to exploit
their remarkable features. However, comparatively little attention has been
paid to the incorporation of CNTs into amorphous glasses and partially
crystalline glass-ceramics to produce silicate-based composites. The avail-
able literature data on these brittle matrix systems indicate that the primary
aim of incorporating CNTs in glass and glass-ceramic matrices has been to
achieve an increase in fracture toughness. However, the interest in these
composites has also led to the investigation of other mechanical and
functional properties; very recently, some of the more technological
properties have been investigated. Although the field of CNT-glass/glass-
ceramic matrix composites is still in its infancy, it is opportune to review
current knowledge in this field and to highlight the potential applications of
these novel composites relevant to future research. These considerations
motivated the preparation of this chapter.
7.2
Carbon nanotubes
The classic allotropes of carbon, diamond, graphite and amorphous carbon,
known for centuries, have recently been augmented by new families of pure
carbon structure. Both spherical (Kroto et al., 1985) and tubular (Iijima,
1991, Radushkevich and Lukyanovich, 1952) versions of fullerenes have
attracted great interest, especially the latter. The unique structure and
properties of CNTs have attracted more attention than any other
nanomaterial in the last two decades and extensive research has explored
the potential for their use in a wide range of scientific and engineering fields.
7.2.1 Structure of carbon nanotubes
Carbon atoms are held together by sp
3
hybrid bonds in diamond. The
presence of four strong sigma covalent bonds constructs a tetrahedral
structure producing hard but electrically insulating diamond. In contrast,
graphite has a layered atomic structure, wherein carbon atoms in each
graphene layer are linked through three sp
2
hybrid sigma bonds (Poole and
Owens, 2003). The out-of-plane pi bond (Meyyappan, 2005) is delocalized
and distributed over the entire graphene layer, making graphite conductive
and highly anisotropic. As a result, strong covalent bonding exists along the
graphene layers and weak Van der Waals forces are present between the
